In manufacturing steps wherein semiconductor products such as a semiconductor integrated circuit, a liquid crystal panel, a solar battery panel and a magnetic disk are produced, an apparatus has been widely used which generates plasma due to high-frequency discharges under a rare gas atmosphere and uses the generated plasma to perform various treatments of semiconductor products or display devices.
Although argon has conventionally been used as a rare gas which is applied to such treatments, krypton and xenon have attracted attention in recent years to perform more advanced treatments.
In the field of lamps, although argon has conventionally been used as a filler gas enclosed in a lamp, a high value added product has been produced in recent years wherein krypton or xenon is used therein to reduce electrical power consumption and improve brightness.
On the other hand, xenon, which causes no adverse drug action, has attracted attention in the medical field as an anaesthesia, instead of nitrous oxide, which has conventionally been used.
However, krypton and xenon are extremely rare and expensive from the view point of their abundance ratio in air and complicated steps are required to separate the gases, and therefore, there is a problem in that cost greatly increases when such an expensive gas is used.
In order to enable the use of such a rare gas economically, it is extremely important to separate and collect a spent rare gas at a high collecting ratio and perform cyclic use thereof.
For example, Japanese Unexamined Patent Application, First Publication No. 2006-61831 proposes a method wherein a high value added gas is separated and collected at high concentration and a high collection ratio from a spent gas mixture including such a high value added gas.
This invention proposes a separation and collection method which uses a pressure swing adsorption method (PSA method), wherein steps (a) to (e), which are shown in FIG. 2 and described below, are performed according to a certain sequence which is determined in advance. A device used in the invention includes: a feed gas storage tank; a strong adsorbate storage tank which stores a strong adsorbate; a weak adsorbate storage tank which stores a weak adsorbate; a compressor which compresses a gas included in the feed gas storage tank or the strong adsorbate storage tank; a compressor which compresses a gas in the strong adsorbate storage tank; four adsorption columns consisting of lower columns 10B and 11B, and upper columns 10U and 11U; and valves which are provided at predetermined positions.
(Step a)
A lower column 10B and an upper column 10U are filled with an adsorbent which has strong adsorbability with respect to one component (a strong adsorbate) of a gas mixture and also has weak adsorbability with respect to other components (a weak adsorbate) of the gas mixture. The gas mixture (a feed gas, that is, a gas to which separation and collection is performed) which includes at least a strong adsorbate and a weak adsorbate, that is, includes two or more kinds of components, is introduced into the lower column 10B from a feed gas storage tank 1. Then, a gas in which a strong adsorbate has reduced is discharged from the lower column 10B and is introduced to the upper column 10U, so that a strong adsorbate is adsorbed by the adsorbents of the lower column 10B and the upper column 10U. A weak adsorbate which has passed through the lower column 10B and the upper column 10U is collected by a weak adsorbate storage tank 3 which connects with the upper column 10U. When the step (a) is completed, no adsorption band of a strong adsorbate arrives at the upper positions of the adsorption columns.
(Step b)
From a strong adsorbate storage tank 2 which stores a strong adsorbate, a gas (strong adsorbate) is introduced to the lower column 10B, to which a strong adsorbate has been adsorbed, so that a weak adsorbate remaining in a space of the lower column 10B is sent to the upper column 10U, and a strong adsorbate included in the gas sent from the lower column 10B to the upper column 10U is adsorbed by the upper column 10U. A weak adsorbate which is discharged from the upper column 10U is collected, and is transferred to the upper column 11U in which a step (e) has been completed.
(Step c)
The lower column 10B is decompressed to desorb a strong adsorbate from the lower column 10B, and the desorbed strong adsorbate is collected by the strong adsorbate storage tank 2.
(Step d)
The upper column 10U is decompressed to desorb a gas (a mixture of a strong adsorbate and a weak adsorbate) which has been adsorbed by the upper column 10U, and the desorbed gas is introduced in the lower column 10B and a gas discharged from the lower column 10B is collected in the feed gas storage tank 1.
(Step e)
The weak adsorbate collected in the weak adsorbate storage tank 3 is introduced into the decompressed upper column 10U as a purge gas. A gas discharged from the upper column 10U is introduced in the lower column 10B. In this way, a strong adsorbate is desorbed by displacement at the upper column 10U and the lower column 10B, and a gas (a mixture of a weak adsorbate and a strong adsorbate) discharged from the lower column 10B is collected in the feed gas storage tank 1.
In this way, a weak adsorbate and a strong adsorbate are collected simultaneously at a high concentration and a high collection ratio.
In such a method for separating and collecting a high value added gas wherein a pressure swing adsorption-type gas separation method is used as described above, in order to collect a weak adsorbate and a strong adsorbate effectively, it is important that the interior of the lower column 10B is saturated by adsorbing a strong adsorbate and that a strong adsorbate that has arrived at the upper column 10U is completely adsorbed, that is, it is important to control a position (height) of an adsorption band of a strong adsorbate so that a strong adsorbate is not discharged from the upper column 10U when the step b is completed.
For example, when a strong adsorbate is introduced in succession in the upper column 10U to which a strong adsorbate has not been introduced, an adsorption band (a portion where adsorption of a strong adsorbate is proceeding) is generated therein at a side where the gas is introduced to the upper column 10U. With the passage of time, the adsorption band moves toward the upper end of the column, and on the other hand, a saturation band (a portion where the amount of the adsorbed strong adsorbate reaches saturated level) is generated subsequent to the adsorption band at the side to which the gas is introduced.
Accordingly, operation conditions and the like are generally controlled in the step b such that the interior of the lower column 10B is saturated by adsorbing a strong adsorbate, and the upper column 10U adsorbs a strong adsorbate completely.
By the way, adsorptivity of an adsorbent depends on a temperature. It is known that the lower a temperature of an adsorbent is, the larger the adsorption amount of a gas adsorbed in the adsorbent is.
Accordingly, there is a problem that, if an ambient temperature around an apparatus which is in operation is slightly changed, in particular, if an ambient temperature around adsorption columns is changed and therefore a temperature of the an adsorbent thereof is changed, each adsorption band of a strong adsorbate in the upper column 10U and the lower column 10B, which has moved by the end of the step b, is shifted, and a collection ratio and collection purity of a strong adsorbate and a weak adsorbate, which should be separated and collected, decrease. It is not preferable that, when the step b is completed, a weak adsorbate remain in the lower column, nor a strong adsorbate be discharged from the upper column.
The adsorption band represents an inflection point (band) at which concentration distribution of a component to be adsorbed is suddenly varied.
When an ambient temperature decreases, an amount of a gas adsorbed in an adsorbent increases and speed of an adsorption band of a strong adsorbate which moves in an adsorption column decreases in general. Accordingly, if operational conditions or the like other than said decrease of temperature are the same, a weak adsorbate remains in the lower column 10B in the step b without being transferred to the upper column 10U from the lower column 10B, and the remaining weak adsorbate is collected by the weak adsorbate storage tank 2 in the step c. As a result, purity of a strong adsorbate collected and a collection ratio of a weak adsorbate decrease.
FIG. 3 shows concentration distribution of a strong adsorbate existing in the lower column 10B and the upper column 10U, wherein the distribution is measured after a predetermined time has been passed and the step b is completed. A longitudinal axis thereof represents concentration of a strong adsorbate, and a horizontal axis represents the total value of height of the lower column 10B and the upper column 10U. The figure shows variation of concentration distribution when an ambient temperature around an apparatus decreases.
In the graph, a curve A represents concentration distribution of a strong adsorbate at a standard temperature A, and a curve C represents concentration distribution of a strong adsorbate at a temperature C which is lower than said standard temperature A.
The curve A represents a state wherein the interior of the lower column 10B has been saturated with a strong adsorbate which is adsorbed thereto, and the upper column 10U has completely adsorbed a strong adsorbate.
The curve C represents that a weak adsorbate remains in the lower column 10B without being discharged from the lower column 10B to the upper column 10U.
On the other hand, when an environment temperature increases, a gas amount adsorbed by an adsorbent decreases, the moving rate of an adsorption band of a strong adsorbate increases in the adsorption column, and as a result, a strong adsorbate arrives at the uppermost position of the upper column 10U in the step b. Accordingly, purity of a weak adsorbate and a collecting ratio of a strong adsorbate decrease.
FIG. 4 shows concentration distribution of a strong adsorbate included in the lower column 10B and the upper column 10U, wherein the distribution is measured after a predetermined time has been passed and the step b is completed. The figure shows variation of concentration distribution when an ambient temperature increases.
In the figure, a curve A represents concentration distribution of a strong adsorbate at a predetermined standard temperature A, and a curve B represents concentration distribution of a strong adsorbate at a temperature B which is higher than said standard temperature A.
The curve B shows that a strong adsorbate arrives at the uppermost position of the upper column 10U, before the step b is completed.
In FIGS. 3 and 4, separation at a temperature C or B is performed using operational conditions or the like which are set according to the standard temperature A.
A separation apparatus may be installed in a room, in which an air conditioning device is provided and controls a temperature uniformly throughout the year, in order to prevent decrease of collection purity and a collection ratio of a strong adsorbate and a weak adsorbate wherein such a decrease is caused by aforementioned variation of an ambient temperature, and to achieve stable separation ability throughout the year.
However, large energy is required for initial cost and running cost of such a system, and therefore a cost thereof increases.